1. Field of the Invention
This invention relates to the field of fault location in a telecommunications network.
2. Related Art
A conventional public telecommunications network is generally described as having two parts, namely a core network and an access network. The access network is that part of the network which extends from the terminal equipment, for example a customer's telephone or a public call box, to a local exchange. The core network includes the local and trunk exchanges.
In a conventional access network, each communications line is formed from a pair of copper wires. Typically, each pair of wires pass through a series of nodes between the local exchange and the terminal equipment. Examples of such nodes are primary cross-connect points, secondary cross-connect points, distribution points and joints.
Recently, optical fibres have been used to replace copper wires in access networks. In a modern access network, both copper wires and optical fibres are used. Where a communications line consists of an optical fibre, the line will typically pass through several nodes between the local exchange and the terminal equipment. At each node, the incoming fibre from the local exchange is split into a group of outgoing fibres which branch out in various directions. The last part of the circuit to the terminal equipment may still be carried by a pair of copper wires.
Since the components of the access network are those which are most exposed to the elements, this part of the public telecommunications network is particularly prone to faults. In a conventional access network, examples of such faults are disconnection faults, where the communications line is interrupted between the local exchange and the terminal equipment, short circuit faults, for example a short circuit between the two wires of a pair of wires and earthing faults, for example a short circuit between one of the wires and earth. The causes of the faults include physical damage to a node as well as leakage of water into a node.
To enable a network provider to remedy faults rapidly, local exchanges are provided with line testing apparatus which may be used to test each line. Such tests can be used to determine the approximate location of the fault between the local exchange and the terminal equipment.
For example, International Publication No. WO96/19072 discloses a method and apparatus for testing telecommunications lines by measuring the line capacitance. A capacitance measuring device at a local exchange periodically tests the capacitance of each wire pair. A capacitance value is stored for each wire pair, denoting the normal operational state of the line. If, during a subsequent measurement, the capacitance changes to an extent which indicates a fault on the line, the value of the capacitance change is determined, from which an approximate distance to the fault is calculated, based on a known capacitance/length ratio for the line. This calculation indicates the distance of the fault from the subscriber's terminal equipment.
The above described system indicates the distance of a fault along a length of line. However, it does not indicate the physical location of a fault, since the line between the terminal equipment and the local exchange follows the pattern of the streets in the area. Therefore, the physical location of the fault can only be determined by having an accurate map of the path followed by the line, which may extend for many kilometres, and measuring the indicated fault distance along this map.